There are known in the prior art many devices for measuring electrical current flowing in a conductor. These devices generally provide a closed, magnetic flux conducting path about a portion of the conductor, and include secondary windings about the flux conducting path in which a voltage is induced by the inductive effect of the conductor on the flux conducting path. The flux conducting path often comprises a laminated core construction often employed in transformer designs.
The prior art current transformers described above all suffer from a source of error which is apparently inherent in the geometry of current transformers. That is, the current transformer must form a closed, flux conducting path about the conductor, yet it often must be portable and therefore removable from the conductor. To fashion the transformer so that it is removable from the conductor, the core laminations must be openable to admit and remove the conductor from within the closed loop core. Thus a portion of the core often is hinged, and is provided with a latch to releasably secure the hinged portion. However, it has been found that the degree to which the hinged portion is closed upon the remaining portion of the core has a pronounced influence on the inductive effect produced in the core by the conductor. If the junction of the hinged portion and the fixed portion of the core does not constantly provide the same flux conducting effect as the remainder of the core, the readings obtained by the instrument will contain a significant error.
Furthermore, the secondary windings about the core of a current transformer are subject to a large voltage surge when the hinged core element is first closed upon the fixed core element. This voltage surge is connected directly to the readout terminals of the current transformer, and comprises a danger to the instrument technician which is dangerous or lethal.